1. Field of the Invention
This invention relates, in general, to radar systems and, more specifically, to radar target extractors.
2. Description of the Prior Art
Target extractors for radar systems include signal processing circuitry which determines various information about the location and projected location of a target illuminated by the radar system transmitter. Electronic target extractors are often used in conjunction with a visual display which indicates the pulses reflected by the target. However, with modern weapon systems, the visual display, if used, rarely is sufficient to determine the needed information accurately and quickly enough. Therefore, an electronic target extractor is normally used to ascertain the information needed about the location, direction, speed, etc. of the target.
In a typical monostatic radar system where the illuminating transmitter and the reflected signal receiver are located at substantially the same position, a Plan Position Indicator (PPI) is generally used. With such an indicator system it is necessary to determine the distance or range to the target from the radar antenna and the direction or azimuth from the radar antenna to the target. While measuring distances and plotting angles on a conventional PPI display screen would roughly give the information needed, the lack of accuracy and the time required could not be tolerated with modern weapon and counter weapon systems. Therefore, efforts have been made in the prior art to "extract" or process electronically the radar signals to achieve a workable system for determining the desired information about the target.
The fact that radar transmissions consist basically of transmitted pulses gives the reflected signal at the radar receiver the characteristics of a pulse signal. When a pulse received by the radar system exceeds a predetermined threshold value, a "hit" is detected by the target extractor. To reduce the effect of extraneous noise pulses, a target is declared only when several close hits are detected by the target extractor. The occurrence of these hits is synchronized to the antenna orientation to provide the azimuth of the hit and are related to the total propogation time of the radar signal to provide the distance or range to the source causing the reflected hit signal.
Due to the multiplicity of hits detected by the target extractor and also to the filtering and sampling circuitry of the radar receiver system, and to the finite antenna beam width, hits are always produced at more than one specific azimuth and range location. Consequently, the extractor must be able to determine a specific azimuth and range location from a field of hits. According to prior art techniques, the azimuth of the target is determined as the angle to the location of the radial hit region or range cell which contains the greatest number of hits. The hits in this "primary" range cell are processed according to other prior art techniques to determine the azimuth of the target.
Two examples of azimuth determining circuitry arrangements frequently used are known by those skilled in the radar art as up-down counter detectors and as sliding window detectors. An up-down counter detector is described in U.S. Pat. No. 3,359,442, issued on Dec. 19, 1967. U.S. Pat. No. 3,646,588, issued on Feb. 29, 1972 describes apparatus for predicting and correcting radar data by "logically processing" a group of bits which correspond to video signals detected by the radar receiver.
Since the overall performance of a radar system is based to a large extent upon the accuracy of the target location provided by the radar extractor, it is important that the accuracy of a radar system be as great as possible consistent with system reliability and economy. Therefore, it is desirable, and it is an object of this invention, to provide a radar target extractor having improved range accuracy over prior art techniques and being compatible with existing azimuth extracting techniques.